A typical construction for a large truck is based on a steel chassis frame. A cab for the driver is mounted atop the frame. An engine is mounted on the frame in front of the cab. Various components of a drivetrain that couples the engine to driven wheels of a rear axle assembly are supported on the frame, as are components of various other systems such as brake and steering systems. In the case of a highway tractor, a fifth wheel is mounted atop the rear of the frame.
One or more rear drive axles are suspended from the chassis frame by a suspension system that comprises various components that provide resiliency in conjunction with damping. The spring/damping characteristic of a suspension system is typically selected to provide desired ride and handling for a particular vehicle use vocation, and is based on the mass of the truck and the maximum load that the truck carries. Truck suspension systems often include components whose characteristics are adjustable in order to better adapt the truck to different load masses.
Assembly of axles, especially tandem axles, to chassis frames requires manufacturing processes that can assure proper alignment. In an assembly line process, some trucks will have more precise alignment than others due to tolerance variations. Imprecise alignment of the rear axle of a tandem axle can create an off-center thrust angle, a condition sometimes referred to as “dog tracking”, that a driver corrects by turning the steering wheel off-center. This contributes to steering wheel misalignment and leads to accelerated tire wear. The presence of a scrub angle between front and rear axles of a tandem axle is another undesired condition.
Trucks are typically manufactured by fastening individual components to a chassis frame using various brackets, cross members, and fasteners. Large numbers of holes have to be drilled in side rails of the chassis frame. Such a manufacturing process is typically part- and labor-intensive. It also requires a number of different assembly stations for which ample floor space in an assembly plant is required.
Certain trailers of the type that are hauled by highway tractors have rear axle modules, sometimes referred to as bogies, or sliders, that contain multiple axles and various suspension components that mount the axle(s) on a structure that is itself fastened to the underside of the trailer body. The use of a modular axle assembly in trailer manufacture can provide certain manufacturing efficiencies because of fewer operations and the ability to better align each axle to the other in the module, and they are assembled in significantly smaller main line workstations that are 12′×12′, more or less, rather than 12′×50′, more or less, often found in conventional highway tractor main line workstations. This results in considerable manufacturing labor and floor space savings.